The measurement of material thickness by the use of ultrasonics is a common inspection technique. Generally, there are two types of ultrasonic thickness measurement techniques comprising the resonance method and the pulse-echo technique. Resonance tests are limited in application, particularly in the tubing industry, because of the low measurement sampling rates that are used by instruments employing this technique. Use of the pulse-echo thickness measurement technique, is more prevalent today than the resonance method primarily because of its higher measurement sampling rate.
An air gage technique is commonly employed for measuring inside and outside tube diameters. Although the accuracy of this technique is high, it is quite slow and requires an internal probe, which may scratch the tube. It is not possible to measure wall thickness by the air gaging technique.
One technique which is used to perform the inside and outside diameters and wall thickness measurements simultaneously relies on the use of capacitance probes. This particular technique also requires contacting an inside probe to measure the internal diameter of the tubing. Generally, such a probe would have a tendency to scratch the tube surface and also restricts the speed at which the tube can be rotated while passing the transducers. Measurement of the wall thickness requires maintaining a very precise relative alignment between the inside and outside capacitance probes while the tube is conveyed past the probe. This is exceedingly difficult, particularly if the tube is rotated at a relatively high rpm.
Other disclosures of prior art ultrasonic techniques primarily used for measuring thickness of a workpiece are typified by the disclosure in the Berg et al U.S. Pat. No. 3,554,014. Other patents considered in connection with this invention include: Zemanek et al., U.S. Pat. No. 3,426,585; Mitchell et al., U.S. Pat. No. 3,474,664; Weinbaum, U.S. Pat. No. 3,599,478.